The present invention relates to a process of producing diffraction gratings for use in distributed feedback (DFB) semiconductor lasers and more particularly to a process of producing a diffraction grating having an intermediate portion such as a quarter-wavelength shift portion where a phase is reversed.
FIGS. 1(a) and 1(b) show a conventional process of producing a diffraction grating, disclosed in, e.g., Technical Research Report OQE85-11 of Japan Electronic Communication Society or in a publication "Optical Integrated Circuits", p216, by Nishihara et al, Ohm Ltd. 1960. In FIGS. 1(a) and 1(b), there is shown a substrate 1 made of semiconductor material, together with a photosensitive resist 2 in the form of a diffraction grating. More specifically, FIG. 1(a) shows the photosensitive resist patterned into the form of a diffraction grating by the two-luminous-flux interference exposure method, whereas FIG. 1(b) shows the substrate 1 etched into the form of a diffraction grating while using the resist 2 as a mask.
Referring to FIGS. 1(a) and 1(b), the conventional process of manufacture will subsequently be described. The photosensitive resist is first applied onto the substrate 1 and the two-luminous-flux interference exposure method is employed to subject the resist to the exposure of interference fringes. Then, the resist is developed. As a result, the resist 2 patterned into the form of a diffraction grating is formed on the substrate 1 as shown in FIG. 1(a). Subsequently, the substrate 1 is etched with a mask of the resist 2 to form the diffraction grating as shown in FIG. 1(b).
In the conventional process of producing a diffraction grating, the interference exposure method simultaneously using positive and negative resists, the asymmetrical interference exposure method using a phase shift membrane, etc. must be utilized to produce the diffraction grating having a phase shift such as quarter-wavelength shift, which causes disadvantages that the process becomes complicated, interference exposure conditions becomes severe and so forth.